Abstract (inglese)

Some possible interactions between the hydrological, geomorphological, and ecological features and processes have been studied here at different spatial scales and resolutions in ecosystems. The ecohydrological framework is to perform an interdisciplinary research to detect with essential models the broad and complex patterns in biological, ecology, geomorphology and hydrology of river basins, in which water plays a key role.
Starting from the evidence of the invariance and universality of some geomorpho- logical patterns in river basins, such as the drainage area and the Hack's lengths, the distribution of the distances between tributaries along the mainstream has been stud- ied using digital terrain maps of real basins, that is directly related to the availability of water and of the channel and riparian area. Specifically it has been found that the probability of exceedence of of the alongstream distances of tributaries larger than a given threshold, has an exponential form in function of the boundary conditions and the Hurst exponent of the basin. Also theoretical constructs have been used such as optimal channel networks, Peano and Scheidegger networks at different growth stages, deriving exact analytical expressions for the probability of the distances between subbasins on a rectilinear mainstrem. The concepts of scale as extension, resolution and coarse-graining level have been introduced in order to study the properties of patterns resulting from ecological and geomorphological processes when varying these quantities.
Biodiversity patterns have been extensively simulated using two principal dispersal mechanisms with a neutral metacommunity model. The mechanism with exponential kernel dispersal and the mechanism with exponential plus a power-law kernel dispersal that ensures a long-distance dispersal to the species. Because the neutrality hypothesis all the individuals compete equivalently in the speciation and dispersal dynamics. The dependence on the scale, resolution and coarse-graining at which the ecosystem is sim- ulated has been investigated for different biodiversity indicators, such as the local and regional species richness, the similarity in species richness between local communities, the species-area relationship and the probability distribution of cluster-size of conspe- cific individuals. Also empirical patterns of fishes and trees (with two classes of diameter resembling two distinct life-stages) of the Mississippi-Missouri River System have been analysed. The model is able to describe the probability distribution of the cluster-size further other patterns as already evidenced by previous studies. The influence of the topology of the ecosystem (2-D landscape and river network), of the shape, and of the en- vironmental heterogeneities, have been analysed in detail. In general all the biodiversity patterns result dependent on the scale and on the resolution. Therefore the species-area relationship results invariant across coarse-graining levels.
The macroecological description of the spatial distribution of the species results re- produced by the neutral model optimally. An interesting relationship has been found between the exponent of the species-area relationship and the exponent of the power-law of the cluster-size for different topologies and dispersal cases. The exogenous (immigra- tion and environmental variables) and endogenous factors (speciation and topology) have been disentangled in order to determine their contribution in the spatial patterns of fish and vegetation. The speciation-death and dispersal phenomena are the determinants in the clustering process without any need to include positive feedbacks between species in order to reproduce the spatial patterns. The environmental heterogeneities, for example dictated by the climate influence the structure of the power-law of the cluster-size of species only in conditions of dispersal limitation and saturation of the local communi- ties. It has been put emphasis also on the species-individual relationship as a tool to better forecast the effect of climate change than the species-area curve, and to possible allometric relationships between the dispersal parameter and some species traits such has mass and lifespan. The relationship between the exponent of the species-area and the lifespan of species is in agreement with what found in literature.
The interactions between the geomorphic structure of ecosystems and the processes acting on them at different scale are of fundamental importance in the management of ecological and water resources. This is the reason why it is extremely important to understand the scale at which the relevant processes take place. It has been evidenced clearly the striking features of the geomorphic supports and external drivers on ecological and biological processes than on the hydrologic dynamics.
Keywords: biodiversity patterns, ecohydrology, scales, river basins, dispersal, neu- trality, metacommunity, landscape ecology, geomorphology.